The long-term goal of this project is to elucidate the signaling mechanisms underlying new synapse formation in the fruit fly Drosophila. We have recently discovered that the secreted glycoprotein Wingless (Wg), best known for its crucial role in early morphogenesis and pattern formation, is also a fundamental organizer of glutamatergic synapses in the fruit fly. In this project we will use genetic, molecular, and electrophysiological strategies to investigate the ways in which the Wg pathway promotes synapse formation. In Aim 1 we will focus on the Wg receptor - Dfrizzled2 - to discern how Wg transduction affects maturing terminals, and if the structural disruptions found on both sides of the synapse in mutants are independent or require antero/retrograde signaling. In Aim 2, Electrophysiological recordings and dye uptake experiments will probe fundamental questions in synaptic transmission such as the role of the active zone in exocytosis, the relationship between endo/exocytosis, and mechanisms for compensation of synaptic strength. Finally, in Aim 3 we will use genetic and yeast two-hybrid approaches to determine the specific signal transduction pathway activated by Wg during synapse development. In particular, we will test the hypothesis that at synapses Wg activated a non-canonical pathway. We will also uncover new proteins that bind directly to Dfrizzled2 and that may function to target and cluster it to synaptic sites. The proposed experiments will fundamentally advance the field of synapse development by characterizing a secreted protein, which is essential for setting up active zones and postsynaptic specializations. Our findings may also bring insights into mammalian synapse development, as these fly synapses show a tantalizing degree of molecular conservation with mammalian central synapses. Therefore our results with manipulating components of the Wg pathway could be important for deciphering the mechanisms underlying a number of neuropathologies, as well as to design strategies to repair nervous system damage after stroke, trauma, or disease. A notable example is the case of bipolar disorder, which has long been treated with lithium. Recent studies show that one of the targets for lithium is GSK3-8, a crucial enzyme in the Wg pathway. Our finding that the Wg pathway is essential for synapse development might provide new insights into this disease. [unreadable] [unreadable]